There are multiple coax outlets on the same wall in my apartment. How do I tell which is Comcast's and which is the building's antenna?

You could open the wallplates and see if there is any ID inside, you can then access your building's electrical/utility room and see if the wiring for Comcast is identified there with your apt number ( match the cable identification with the wiring inside your wallplate ). Or you could just ask your apartment complex maintenance staff or the Manager,

You can also just connect the STB and TV to one of them and see if your cable service is active.

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It would be faster to connect with a coaxial-cable to the antenna input on the television. Then scan for stations. If it finds some, connect the cable-box to the other one. The OTA channels will give you some back-up when the cable fails.

Can you see any signs of an antenna or satellite dishes on the roof? Any signs of cable-box connections to main trunk cable outside of the building?

I'm certain that there are all 3 coming into the building and that there are coaxial outlets for the building's antenna &/or Comcast or Direct TV in each of the apartments. I just don't know which is which. Thanks for the help. I'm planning on testing them tomorrow or Monday.

I suspect you may have to have Comcast service personnel come out to get the cable activated and connected to their trunk cable to install your initial service in your apartment. Replacing a faulty box at a later date most likely can be done by self install.